1. Field of the Invention
The field of art to which this invention pertains is purification of diesters of phenylene dicarboxylic acids, in particular, the purification of the diesters of terephthalic or isophthalic acids, by an oxidation treatment of mother liquor in conjunction with evaporation treatment of the oxidized mother liquor, thereafter a bottoms stream of high boiling organics and ash is recovered, and a low boiling overhead material is returned to the esterification zone.
2. Description of the Prior Art
Relevant prior art includes Japanese Patent Publication No. 15016/65 (Application No. 55066/62). Disclosed in this publication is an improvement in a process for producing terephthalic or isophthalic, acid diesters in which material such as (1) a residual liquid obtained by further distillation or concentration of a mother liquor which has been separated from the crystallized diester and/or (2) the low boiling distillate which is obtained from the distillation of the crystallized diester, is oxidized and then returned for recirculation into an esterification zone. The primary impurities described in either the concentrated mother liquor or the low boiling material removed from the diester crystals is thought, according to the Japanese Publication, to be a formylbenzoic acid methyl ester which upon oxidation forms a mono methyl ester of terephthalic or isophthalic acid.
In the subject Japanese Publication, a portion of the mother liquor is subjected to a treatment for decolorization by means of distillation or the like and then delivered directly to the esterification zone as recycle without oxidation treatment. The portion of the mother liquor into which terephthalic or isophthalic acid diester has migrated or has been concentrated contains a significant amount of impurities (formylbenzoic acid methyl ester) and is delivered to an oxidation reactor and then returned to the esterification reaction zone.